Receiver circuits



B. D. H. TE'LLEGEN I 2,279,185

April 7, 194 2;

RECEIVER CIRCUITS Fi-l'e'd Sept. 27, 1939 I W E IV TOR. as??? 1 rzz-ue'sslv 2 Sheets-Sheet 2 (Lu-v61 ATTORNEY.

Patented Apr. 7, 1942 1 RECEIVER CIRCUITS Bernardus D. H. Tellegen, Eindhoven, Netherlands, assignor, Corporation of by mesne assignments, to Radio America, New York, N. Y., a corporation of Delaware Application September 27, 1939, Serial No. 296,717 In the Netherlands October 15, 1938 7 Claims. (Cl; 250-20) This invention relates to a circuit arrangement I for accentuating either the carrier wave with respect to the side-bands, or the side-bands with respect to the carrier wave of an amplitudemodulated oscillation.

In some cases it is necessary that the side-bands of an amplitude-modulated oscillation should be accentuated with respect to the carrier wave. Such case occurs, for example, with test oscillators, in which an amplitude-modulated oscillation must be generated with a modulation depth of 100% and with a very low distortion of the modulation. For this purpose it is frequently desirable that the generated oscillation should first be modulated with a'very low modulation depth, say 10%, and. that then the modulation depth should be increased by accentuating the sidebands with respect to the carrier wave. This may be effected, for example, with the aid of very sharp filters, in particular crystal filters. In other cases it is necessary that the carrier wave of an amplitude-modulated signal should be accentuated with respect to the side-bands, for which purpose also use may be made of crystal filters.

The use of crystal filters for the above-mentioned purposes is very expensive. Moreover, due to the fixed tuning frequency of crystal filters the latter can be used only for a single carrierwave frequency. Another disadvantage inherent in the use of crystal filters is that the carrier wave of the oscillation, of which the modulation depth must be influenced, must be maintained constant with great accuracy, for which purpose in many cases a complicated and accurately operating device for automatic frequency correction is required.

The present invention has for its purpose to provide a circuit arrangement for influencing the modulation depth of an amplitude-modulated oscillation, in which the above mentioned disadvantages are avoided.

According to the invention, use is made of a network constituted by the parallel-connection of a rectifier operating with peak detection, of which the output impedance for alternating currents of the modulation frequencies is substantially difierent from the output resistance for direct current, and an oscillatory circuit which is tuned to the carrier-wave frequency.

The amplitude-modulated oscillation of which the modulation depth must be influenced, can be fed to this network in difierent manners. According to one embodiment of the invention, the network has a current supplied to it which is v high internal resistance, to

proportional to the oscillation of which the modulation depth must be influenced, which may be eflected, for example, by interposing the network in the anode circuit of an amplifying tube of the control grid of which tube is fed the said oscillation.

7 According to another embodiment of the invention, a voltage proportional to the oscillation of which the modulation depth must be influenced, is induced in the oscillatory circuit which is connected in parallel with the rectifier.

The output voltage of the circuit arrangement may be constituted either by the voltage produced across the network'or by the voltage produced across a self-inductance coil which is inductively coupled to the oscillatory circuit connected in parallel with the rectifier. In both of these cases the output impedance of the rectifier for alternating currents of the modulation frequencies must be given a low value for accentuating the carrier 'wave, and for accentuating the side-bands it must be given a high value with respect to the output resistance for direct current.

. The output voltage of the circuit arrangement can also be'derived from a second oscillatory circuit which is tuned to the carrier wave frequency and inductively coupled to a self-inductance coil which is connected in series with the rectifier. In this case the output impedance of the rectifier for alternating currents of the modulation frequencies must be given a high value for accentuating the carrier wave, and for accentuating the side-bands it must be given a low value with respect to the output resistance for direct current.

The output impedance of the rectifier is preferably constituted either by the parallel-connection of an ohmic resistance and a condenser, of which the impedance for alternating currents of the modulation frequencies is small with respect to the said resistance, or by the series-connection of an ohmic resistance and a choke coil, of which the impedance for alternating currents of the modulation frequencies is great with respect to the said resistance.

According to another feature of the invention, the blocking resistance of the oscillatory circuit which is connected in parallel with the rectifier,

is given a high value with respect to the output.

resistance of the rectifier for direct current, which may be achieved, for example, by undamping the oscillatorycircuit by means of feedback coupling.

Incontradistinction with the known filters,

comprises a large number of harmonics of the carrier wave frequency. To ensure a satisfactory operation of the circuit arrangement, it is, therefore, necessary that the impedance of the rectifier circuit for the generated harmonics is low with respect to the impedance for the carrier wave frequency. Otherwise voltages with the for which purpose so far use has always been 7 made of limiters.

The invention will be more clearly understood by reference to the accompanying drawings showing, by way of example-different embodi-' ments according thereto in Figs. 1 to 7 inclusive.

Fig. 1 shows an amplifiertube' Jof high internal resistance. The oscillation, of which the modulation depth must be influenced, is fed to terminals 2 and 3 which are connected respectively to the control grid and the cathode of the tube I. The anode circuit of the tube I is consequently traversed by a current which is proportional to the oscillation supplied. The anode circuit comprises a network which is constituted by "the parallel connection of an oscillatory circuit 4 which is'tuned to-the carrier-wave frequency of the oscillationsupplied, and a diode 5 operating with peak detection. A condenser 6 is provided in series with the diode'5, which condenser has a low impedance for-the carrier wave frequency and a high impedance for the modulation frequencies, and constitutes a. parallel con nection with the output impedance 1, of which theohmic resistance is much different from the impedance for'alternating currents of the moclulation frequencies. The voltage produced across the described network is suppliedto output terminals 9 and I througha condenser 8.

The operation of the follows: As can be demonstrated by calculation, the impedance forthe carrier wave frequency of a rectifier operating with peak detection, to which is fed an amplitude-modulated signal, is equal to half the output resistance of the rectifier for direct current; whereas for an arbitrary side-band frequency the impedance is equal to half the output impedance of'the rectifier for the modulation frequencycorresponding to this side-band frequency. A rectifier operating with peakdetection, of which the output impedance for alternatingj currents of the modulation frequencies is substantially'diflerent from the output resistance for direct current, consequently behaves in relation to "an amplitude-modulated signal supplied to'the rectifier, as a bi-pole, of which the impedance for the carrier'wave frequency is much different from the impedance for the side-band frequencies. 1 In other words it behaves as a bi-pole containing'a resonant circuit which is very sharply tuned to the carrier wave frequency, that is to say a parallel resonant circuit in the event thatthe output impedance of the rectifier for'alternating currents of the modulation frequencies is smalL'and a series resonant circuit in the event that the output impedance of the rectifier for alternating currents of the modulation frequencies is great with respect to the output resistance for direct current. The network interposed in the anode circuit of the tube l consequently has 'for the carrier wave frequency an impedance which is substantially different from the impedance 'for the side-band frequencies so that a voltage will be produced across this network; j of which'the modulation depth is substantially different from that of thecurrent supplied to the network.

The current flowingfthrough the rectifier circuit arrangement is as much smaller than thatof the frequency of one or more of the harmonics are produced at the terminals of the rectifier, which disturb satisfactory operation of the rectifier.

This condition is satisfied by the presence of the oscillatory circuit 4 which has a high impedance'for the carrier wave frequency, but a very low impedance for the harmonics of the carrier Wave frequency. In the circuit arrangement illustrated in Fig. 1, the direct current voltage is applied to the anode of the tube I through the coil of the circuit 4. A disadvantage of this manner of feeding is that the cathode of the diode 5 is at a high potential with respect to ground which'may lead to difi'iculties in the supply of heating current forthe diode.

Fig. 2 shows a-circuit arrangement in which this disadvantage is obviated by using parallel feeding with theaid of a 'choke coil II and a blocking condenser l2. 'Moreover, in the circuit arrangement of Fig. 2 the output impedance of the rectifier 5 is constitutedby a resistance I 3, with which the series connection of a large condenser and a resistance I5 is connected in parallel. The resistance -l5 is small with respect to the resistance [3 so that the output impedance of the diode 5 for alternating currents of the modulation frequencies'is low with respect to the output resistance for direct current. Consequently, the diode 5 behaves for the supplied oscillations as a bi-pole containing a parallel resonant circuit which is very 'sharply tuned to the carrier-wave frequency so that the impedance of the network interposed in the anode circuit of the tube is considerably greater for the carrier wave frequency than for the side-band frequency. Consequently, a voltage will be produced across the'circuitf l, of which the modulation depth is oscillation fed to the terminals 2,- 3. A voltage with a correspond inglydecreased modulation depth is induced in the coil l6, which is inductively coupledto the circuit 4, and supplied to the output terminals v current of the diode 5.

9 and I0. By means of this circuit arrangement thecarrie'r wave of the supplied oscillation is accentuated with respect to the side-bands.

' Since a voltage is produced across the circuit 4, of which the modulation depth is smaller than that'of the'anode current of the tube the current suppliedto the circuit F-will also exhibit a smaller modulation depth. Consequently, the current flowing through the diode 5 will exhibit a greater'modulation depth than the anode current of the tube which may lead to diihculties if the voltage fed to'the terminals'2 and 3 is modulated very deeply. In fact, in this case an over-modulation of' the-current flowing through the diode 5 could be produced'with the result that the voltage produced across the condenser 6 would be considerably distorted, thus disturbing the correct operation of -the diode 5. This inconvenience can' be eliminated by giving the blocking resistanceof the circuit 4 a high value with respectto the output resistance for direct In fact, it is thus ensured that practically no current flows through the'circuit' 4 so that "the whole anode current of thetube l isled throughthediodei, andthe current flowing through the diode 5 consequently cannot have a greater modulation depth than the anode current. This condition can be satisfied, for example, by reducing the damping of the circuit 4 by means of feedback coupling, as is shown in detail in Fig. 3.

In the circuit arrangement of Fig. 3, which also serves to accentuate the carrier wave with respect to the side-bands, the output impedance of the diode 5 is constituted by the parallel connection of a resistance l3 and a condenser H, which latter condenser exhibits a very low impedance for alternating currents of the modulation frequencies, this in contradistinction with the usual rectifier circuit arrangements, in which the condenser H has a high impedance for alternating currents of the modulation frequency. While in the usual rectifying circuit arrangements the condenser I! usually has a capacity of the order of 100 mmf., the capacity of the condenser I! in a circuit arrangement of Fig. 3 may be approximately 1 mi. A very high blocking resistance of circuit 4 is obtained by a positive feed-back coupling with the aid of a coil l8 which is inductively coupled to the circuit 4 and interposed in the control grid circuit of the tube I in serics with the terminal 2 and 3.

For accentuating the carrier wave with respect to the side-bands, use may also be made, without objectio-n,'of the circuit arrangement of Fig. 4

when a positive feedback coupling is used for undamping the oscillatory circuit 4, said circuit ar' rangement being distinguished from the arrangement of Fig. 3 in that the diode 5 and condenser H are mutually exchanged. This circuit arrangement in itself has the disadvantage that the circuit 4 is more strongly damped by the resistance l3. This inconvenience can be eliminated, however, by increasing the feedback coul Fig. 5 shows a circuit arrangemenaby means of which the side-bands of an amplitude-modulated signal can be accentuated with respect to the carrier wave and by means of which, for example, the modulation depth of a signal which is fed to the terminals 2 and 3 and modulated very slightly, can be increased upto 100%. The oscillation, of which the modulation depth must be increased, is fed to the network 4, 5 by inducing in the circuit 4 a voltage which is proportional to this oscillation. This is eifected by coupling this circuit inductively to an oscillatory circuit 2| which is interposed in the anode circuit of the tube I. As in the arrangement of Fig. 3, the output impedance of the diode 5 is constituted by the parallel connection of a resistance I3 and a condenser I! of great capacity. The blocking resistance of the circuit 4 has been given a lower value, however, so that the diode 5 is traversed by a current, the modulation depth of which is considerably greater than that of the anode current of the tube I. The output voltage naL i'n which use is made of a rectifier operating with'peak detection, of which the output impedance for alternating "currents of the modulation frequencies is great with respect to the output resistance for direct current. In fact, the

output impedance of the diode '5 is constituted in for the oscillation fed to the terminals 2 and 3 behaves as a bi-pole comprising a series resonant circuit which is sharply tuned to the carrier wave,

frequency. The condenser 6 in this circuit arrangement acts as in the circuit arrangements of Figs. 1 and 2, as a low impedance for the carrier wave frequencies, but as a high impedance for alternating currents of the modulation frequencies. As in the circuit arrangement of Fig. 5, the oscillation, of which the modulation depth must be increased, is fed to the network 4, 5 by inducing in the circuit 4 a voltage which isproportional to this oscillation, the circuit 4 being inductively coupled to a coil 23 which is interposed in the anode circuit of the tube l. Furthermore, it can be mentioned that the resistance I3 is directly connected in parallel with the circuit 4,,

with the result that this circuit will be rather strongly damped. In the present circuit arrangement this does not involve great difficulty, however, since it is not necessary to satisfy the'condition that the blocking resistance of the circuit 4 is high with respect to the output resistance of the diode 5 for direct current.

In conclusion, Fig. '7 represents a circuit arrangement in which the output impedance of the rectifier 5 for alternating currents of the modulation frequencies is high with respect to the output resistance for direct current, but in which is derived from an oscillatory circuit I9 which is however, by the circuit l9 to such an extent that practically no harmonics are fed to the output terminals 9 and In.

Fig. 6 represents another circuit arrangement for accentuating the side-bands with respect to the carrier wave of an amplitude modulated sigthe carrier wave is accentuated with respect to the side-bands. The output impedance of the diode 5 isconstituted by the series connection of a resistor l5 and a choke coil 22, of which the impedance for alternating currents of the modulation frequencies is great with respect to the re sistance l5. The condenser 6 constitutes a high impedance for alternating currents of the modulation frequencies. As in the circuit arrangement of Fig. 6, the diode 5 behaves for the oscillation fed to theterminals 2 and 3 as a series resonant circuit which'is sharply tuned to the carrier wave frequency. If the blocking resistance of the circuit 4 is not given too high a value, the diode 5 will be traversed by a current of which the modulation depth is smaller than that of the anode current of the tube I so that an alternating voltage of small modulation depth can b derived from theterminals 9 and In.

As may appear from the foregoing the net workused according to the invention has the particular feature that it behaves as a resonant circuit which is automatically tuned exactly to the carrier wave frequency. With respect to the use of crystal filters this feature offers the important advantage that an accurately operating device for correcting the carrier wave frequency can be dispensed with. Moreover, said feature creates the'possibility of using the "circuit arrangement according to the invention for as oscillations with the same phaseor frequency modulation but with a substantially constant amplitude may be taken from the terminals 9 and I 0. Instead of a diode,'use may alsobe made, of another rectifierwith two electrodes, for ex-' ample, a dry rectifier.

What is claimed is; 1. -In combination with an electron discharge tube having input and output electrodes, said input electrodes being adapted to have signalmodulated carrier voltage appliedthereto, a signal-modulated carrier voltage output, network coupled to the output electrodes, a second network connected between the output electrodes for controlling the effective percentage modula-'- tion of the applied modulated carrier voltage comprising a peak detection rectifier and a resonant circuit, said resonant circuit being'tuned to the carrier frequency, a resistor element in a closed: series circuit with said rectifier and resonant,,circuit, said resistor element being free of rectified currents therethrough and providing an impedance for alternating currents. of the modulation frequencies which is substantially dif ferent from the resistance for d irect current, and said output network utilizing the controlled modulated carrier voltage developed across said reso-. nant circuit.

2; In combination with an electron discharge tube having input and output electrodes, said im put electrodes being adapted to have signalmodulated carrier voltage applied thereto, a signal-modulated carrier voltage output network coupled to the output electrodes, asecond network connected between the output electrodes for controlling the effective percentage'rnodulaimpedance' for alternating currents of the modulation frequencieswhich is substantially different from the resistance for direct current, and said output networkutilizing the controlled modulated carrier voltage developed across said resonant circuit. I I

4. In combination with an electron discharge tube havinginput and output electrodes, said input electrodes being adapted to. have signalmodulated carrier voltage applied thereto, a signal-modulated carrier voltage output network 7 coupled to the outputelectrodes, a second nettion of the applied modulated carrier voltage comprising a peak detection rectifier and a resonant circuit-arranged in parallel with each other, aresistor element in 'a closed series circuit'with said rectifier and resonant circuit, said resistor element being free of rectified currents therethrough' and providing an impedance for alternating currents ofthe modulation frequencies which is substantially different from the resistance for direct current, said resonant circuit being tuned to the carrier frequency, and said output network utilizing the controlled modulated carrier .voltage developed across said-resonant circuit. 7 l j i 3. In combination with an electron discharge tube having input and output electrodes and having a relativelyhigh-internal resistance, said input electrodes being adapted to have signalmodulated, carrier voltage applied thereto, a signail-modulated carrier voltage output network coupled to the output electrodes, 2. second network connectedbetweflen the output electrodes for controlling the effective percentage modular-- tion of the applied modulated carrier voltage comprising a peak detection rectifier and a resonant circuit, said resonantcircuit being tuned'to work connected between the output electrodes for controlling the effective percentage modulation of theapplied modulated carrier voltage comprising a peak detection rectifier and a resonant circuit, said resonant circuit being tuned to the carrier frequency, and said output network utilizing the controlled modulated carrier voltage developed acrosssaid resonant circuit, said rectifier including in circuit therewith a load resistor, and the impedance of said load for alternating currents of modulation component frequencies being different than the ohmic resistance thereof.

5. In combination with an electron discharge tube having input and output electrodes, said input electrodes being adapted to have signalmodulated carrier voltage applied thereto, an output network coupled to the output electrodes, a second network connected between the output electrodes comprising a peak detection rectifier and a resonant circuit, said resonant circuit being tuned to the carrier frequency, and said output network utilizing the modulated carrier voltage developed across said resonant circuit, and means for, reactively coupling said resonant circuit to said input electrodes thereby to provide feedback to the latter.

6. In a signal-modulated carrier voltage transmission system of the type having carrier voltage input terminals and output terminals; the improvement which comprises a network connected between the output terminals for controlling the effective percentage modulation of the applied modulated carrier voltage and which has a substantially different impedance for the carrier frequency than for the modulation component frequencies, said network including a resonant circuit tuned to said carrier frequency, reactive means regeneratively coupling'said resonant circuit to said. input. terminals, and a diode rectifier circuit operatively associated with said resonant circuit, said rectifier circuit comprising a load elementwhose impedance'for modulation component frequency currents is different from its 7 ohmic resistance.

'7. In a signal-modulated'carrier voltage transmission system of the typehaving carrier voltage input terminals and output terminals; the im-- provement which comprises a network connected between the output terminals for controlling the effective percentage modulation of the appliedmodulated carrier voltage'and which has a sub-.

nant circuit, said resistor element being free :of

rectified currentstherethrough and providing an stantiall'y different impedance for the carrier frequencythan for'the modulation component frequencies, said network including a resonant circuit tuned to said carrier frequency, means reactively coupling said'resonant circuit to said input terminals, and a diode rectifier circuit op-' e'ratively associated with said'resonant circuit,

said resonant circuit having a high impedance for currents of carrier frequency, but a low im pedance for harmonics of the latter;

. BERNARDUSD. H. 'I'ELLEGEN. 

